1. Field of the Invention
The present invention relates to a method of producing hardened Al-Mg alloy sheets and coated hardened aluminum alloy sheets which have high levels of strength and formability and which have been used in easy-open can ends or the like.
More particularly, the present invention is directed to a method of producing hardened aluminum alloy sheets which are significantly improved in both resistance to intergranular corrosion (pitting corrosion) and bend ductility together with having a combination of high strength and good formability.
2. Description of the Prior Art
Conventionally, in making easy-open type can ends, there have been employed work-hardened sheets fabricated from aluminum alloys including Mg as a primary alloying element e.g. AA 5082, AA 5182 or the like, in which cold rolling has been practiced to obtain an increased strength and, further, baking of a corrosion-resistant coating applied onto the sheets. The conventional work-hardened sheets used in such applications contain Mn, Zr and V in orer to compensate for the strength loss caused due to the coating and baking operations. (Japanese Patent Publication No. 57-33332). Further, there is also proposed another fabrication process in which the sheets are hot rolled and, if desired, cold rolled. Thereafter, the sheets are subjected to an intermediate annealing at temperatures of 300.degree. to 400.degree. C. and a cold rolling to impart an increased strength to the resulting work-hardened sheets. During the coating and baking operations, distortion occurs due to the residual strain in the sheets, thereby presenting serious problems in subsequent operations. A method to relieve such residual stress is proposed in Japanese Patent Publication No. 57-11384 in which heat treatment (stabilizing treatment) is conducted at temperatures of 250.degree. C. or less after a finishing cold rolling.
However, in recent years, there have been a increasing demand for thinner can stock and contents in cans have been more corrosive. In response to the demand for thinner can stock, can stock has been strengthened by increasing the addition of Mg or increasing the reduction amount in the finishing cold rolling step, as set forth above. However, these methods result in a reduced corrosion resistance. Further, the increasing corrosive properties of the contents may cause pitting corrosion and it has been found that even if the can stock is subjected to a stabilizing treatment, in addition to the above treatments, there is still the probability of similar problems. Apparently, in known materials, improvements in the alloys strength and formability adversely affect its corrosion resistance and there has been a problem of how to improve corrosion resistance. Further, an excessive reduction in the amount of finishing cold rolling will lower forming characteristics, such as deep-drawing characteristic (erichsen value) and bend ductility. In some cases, an easy-open pull tab or ring pull attached onto a can end is repeatedly bent or pulled to open the can end, for example, of a juice can. Such an occurrence is not usual but, for example, children try to open cans in such a manner and break the pull tab or ring pull from the repeatedly bent portion before opening the can.
It is therefore an object of the present invention to provide a method of producing hardened aluminum alloy sheets in which their corrosion resistance is significantly improved without lowering their strength and formability.
It has been known from previous studies that Mg, as a strengthening element, bonds to Al to form a compound (.beta.-phase Al.sub.8 Mg.sub.5) which is electrochemically less noble than the matrix. Particularly, when the .beta.-phase is preferentially precipitated along grain boundaries in a can end material, intergranular corrosion proceeds due to the difference in pitting potential between this phase and the matrix, and, thereby, contents within a can will leak. In view of such a problem, conventional can end materials have been investigated and, as a result, it has been confirmed that the above detrimental precipitation preferentially occurs not only along recrystallized grain boundaries formed during the intermediate annealing, but also along grain boundaries during the final stabilizing treatment, thereby lowering the corrosion resistance of the resulting alloy materials. Attempts have been made to overcome such a problem. In order to increase the strength of can materials, addition of Mg has been increased or finishing cold rolling has been effected with a large amount of reduction. However, such a conventional manner is undesirable from the point of corrosion resistance, because it may induce the intergranular corrosion problem.